Resonant sensors are devices which use mechanical vibrations, typically induced at a specific resonant frequency, to detect any of a variety of conditions. Typical implementations include “tuning fork” resonators, piezo-electric resonators, hemispherical resonators, and tilting wheel resonators. In many cases, resonant sensors (such as Micro-Electro-Mechanical Systems [MEMS] sensors) can be compact, low in cost, and highly accurate. Because of the inherent sensitivity of most vibrating systems to physical manipulations, resonant sensors can be used to detect accelerations and rotations, and are often used in accelerometers and in devices which detect rotations (referred to sometimes as “gyroscopes,” although not based on the classic “spinning top” gyroscope approach).
Resonant sensor accelerometers, gyroscopes, and other resonant sensors that are critical to flight guidance algorithms, are frequently used in guided rockets and missiles, which typically have severe vibration environments. Under some circumstances, including the burn of the rocket motor fuel, the body vibrations may induce a resonant frequency at, or close to, the resonant frequency of the sensor, which can interfere with the initialization of the device and can cause the device to provide highly inaccurate data.
In many cases, for example for many air-to-surface missiles, the rocket motor burn lasts for only a short period of time after the initial launch of the rocket, such as a few seconds, after which the rocket glides to its target with relatively low vibrations. However, if the guidance system has been improperly initialized due to vibrations during the initial fuel burn, then the guidance system may provide highly erroneous data, both during and after the burn, and the missile may accept the data from the guidance system as if it were accurate, thereby causing the missile to rapidly deviate from its intended course.
Typically, this problem is addressed by providing shock isolators, which attempt to isolate the resonant sensors from shocks and vibrations experienced by the rocket or missile. However, this approach can be inadequate and/or impractical.
Some guidance systems include a “Built-in-Test” (BIT) fault detector. However, these fault detectors can also be fooled by vibrations of the rocket or missile, and can fail to detect if a resonant sensor has been improperly initialized.
What is needed, therefore, is a system for reliably detecting when a rocket or missile guidance system which includes at least one resonant sensor has been improperly initialized, and providing appropriate corrective action, so as to improve the fault tolerance of the guidance system.